In an effort to eliminate, or to at least minimize, the potential damage to the marine environment caused by oil spills, the International Maritime Organization (IMO) in 1992 adopted new regulations concerning the construction and requirements for the retrofitting of oil tankers. From 1993 onwards, new tanker vessels must be built with a double hull or equivalent arrangements. Tanker ships built prior to 1993 are governed by Regulation 13G of MARPOL 73/78 and must be fitted with a specified double hull or equivalent arrangement when they reach 25 or 30 years of age. It has been estimated that the total number of tankers that continue to transport liquid petroleum cargos is in excess of 2,000, and well over one half of that number were built prior to 1980. Therefore, a significant number of the world's older tanker fleet must meet the requirements of Regulation 13G in order to continue to operate after their 25th year.
MARPOL Regulation 13G mandates that the non-cargo carrying spaces of the tankers in the ultra-large and very large crude carrier U/VLCC category must cover at least 30% of the side of the ship for the total cargo tank length and be at least 2 meters in height, or cover at least 30% of the projected bottom area of the total cargo tank length and have a depth of at least 2 meters or one-fifteenth (1/15th) of the beam of the ship, whichever is less.
Various alternatives have been proposed for meeting the requirements of Regulation 13G. One option is to designate selected cargo tanks as non-cargo carrying spaces. In other words, these tanks will no longer be used for cargo or for ballast. Although this is a relatively simple approach to compliance in one respect, this approach has a significant economic impact since about 20% of the ship's cargo-carrying capacity is lost. In selecting the tanks which are to become non-cargo carrying spaces, care must be taken to insure that the remaining tanks can be loaded for both ballast and cargo that meet conditions that are within the prescribed limits for longitudinal bending moment and sheer forces. As a practical matter, additional cargo space may be lost in order to meet these safety and engineering requirements or, alternatively, expensive bulkhead modifications will have to be undertaken. Additional costs will be incurred in order to insure compliance, e.g., blind flanges will have to be installed in order to insure that the designated tanks are not inadvertently loaded; existing vents and pumps will also have to be maintained to be used in the event of an emergency.
Another option is to provide segregated ballast tanks (SBT) or dedicated clean ballast tanks (DCBT) in order to meet the 30% side or bottom protective location requirements. Although the conversion cost of this alternative is relatively low, the loss of cargo capacity and the attendant reduction in income can amount to several millions of dollars per year at current freight rates for a VLCC tanker.
Another approach to meeting the requirements of Regulation 13G is to install bulkheads to provide the coverage over 30% of the sides and bottoms, with an estimated loss in cargo capacity of about 2.2%. However, the cost of retrofitting a VLCC with bulkheads is estimated to be about $20 million.
One other means of compliance with Regulation 13G is the use of hydrostatically balanced loading techniques. Using this method, the cargo is loaded in all cargo tanks to a level that is less than the freeboard, so that at the cargo tank bottom the hydrostatic pressure of the cargo oil column plus the inner gas pressure in the ullage space remains equal to, or less than, the hydrostatic pressure of the outside water column, thereby mitigating the outflow of oil in the case of bottom damage. Since the hydrostatically balanced loading technique requires no alteration to the ship's structure, pumping or piping systems, the conversion costs are very low. However, the reduction in cargo carrying capacity calculated on an annual basis can result in a loss of about $4 million in freight revenues. In addition, the transport of cargo in partially filled tanks raises the possibility that the natural period of the partial cargo resonating with the nature period of ship motions in a seaway could create substantial sloshing forces and the attendant risks to navigation and to the structural integrity of the ship itself.
Thus, based upon the various analyses that have been undertaken by, or on behalf of the industry, each of the above alternatives proposed for complying with Regulation 13G will incur either very high initial costs, or relatively high annual losses in freight revenues.
It is therefore an object of this invention to provide a construction and method of fabricating a double bottom ship's hull that complies with current regulations, and specifically with MARPOL Regulation 13G, which minimizes the loss in cargo capacity as a percentage of the overall capacity of the ship, thereby minimizing the loss in freight carrying revenues, while also being less expensive over the useful life of the ship than other proposed alternatives.
It is another object of this invention to provide a method of constructing a double hull in an existing oil tanker that will result in a construction that is in compliance with current maritime regulations governing the operation of oil tankers that were constructed more than 25 years ago.
It is yet another object of the invention to provide a method of retrofitting an existing U/VLCC that can be completed relatively quickly and at a relatively low cost.
Another object of the invention is to provide a completed construction and a method for retrofitting existing oil tanker hulls with a double bottom that employs inexpensive, readily available materials which can be fabricated and installed with relative ease, taking into account the confined space and existing structures in the hold of the ship.